FIG. 10 of the accompanying drawings shows the circuit arrangement of a conventional arc-processing power supply system utilizing a full-bridge inverter circuit. As shown in this figure, the conventional power supply system comprises a first through an eighth switching elements TR1–TR8. The third switching element TR3 and the fourth switching element TR4, as power supply switching elements, are alternately brought into and out of conduction to control the supply of DC voltage from an DC power supply circuit which includes an AC power source, a rectifying circuit DR7 and two smoothing capacitors C1, C2. When each of the switching elements TR1, TR2, TR5, TR6 of the full-bridge inverter circuit is turned off, an auxiliary capacitor C5 is charged to a high voltage by the energy of the leakage inductance of a main transformer INT, so that the voltage across the capacitor C5 may exceed the rated voltage of the respective switching elements TR1, TR2, TR5 and TR6 of the full-bridge inverter circuit. The other two switching elements TR7, T8 are surge voltage bypass elements provided for bypassing the charge voltage of the auxiliary capacitor C5 to the first and second smoothing capacitor C1, C2 to prevent the generation of such a high voltage.
The auxiliary capacitor C5 enables each of the first to sixth switching elements TR1–TR6 to switch at zero voltage. After each of the third and fourth switching elements TR3, TR4 are turned off, the auxiliary capacitor C5 continues to discharge a stored charge, and the corresponding ones of the switching elements TR1, TR2, TR5, TR6 of the inverter circuit are turned off when the voltage of the auxiliary capacitor C5 becomes zero, whereby turning-off at zero voltage is possible. Further, by the provision of the auxiliary capacitor C5, the voltage applied to the third switching element TR3 and the fourth switching element TR4 becomes zero, which enables switching of these switching elements at zero voltage.
The conventional arc-processing power supply system additionally comprises a main transformer INT, a secondary rectifier diode DR8, a DC reactor DCL, a secondary current detector ID, a comparison circuit ER, an output control circuit SC, an inverter driving circuit SRC, and a power switching circuit HR, and a power switching circuit HRC. The structure and function of these additional components are fully described in U.S. Patent Application Publication No. 2003/0156435A1, which is incorporated herein by reference.
In the prior-art full-bridge inverter circuit adapted for high voltage, the overall structure is considerably complicated, whereby the size of the system is increased, resulting in an increase in the cost. Moreover, the control circuit becomes complicated because of the complicated structure.